A sample measuring apparatus has been known which measures a sample by applying laser light to a flow of a specimen prepared from the sample. The sample measuring apparatus uses a laser diode as a light source. The laser diode emits laser light of a single wavelength when continuously supplied with a drive current at a constant level. In such a single-mode oscillation state, factors such as a change in temperature of the laser diode cause a phenomenon in which the wavelength of the laser light changes stepwise. Such a phenomenon is called mode hopping.
In a specimen analyzer described in Japanese Patent Application Publication No. 2009-53020 (Patent Literature 1), a high-frequency component is superimposed onto a drive signal for a laser diode to set the laser diode to a multi-mode oscillation state. By adjusting the drive signal in this manner, the laser diode switches on and off in a short cycle, thereby suppressing the occurrence of mode hopping.
Laser diodes sometimes vary in output due to individual differences. In such a case, the specimen analyzer described in Patent Literature 1 uses an APC (Automatic Power Control) circuit to adjust the current of the drive signal for the laser diode such that the laser diode emits light with an expected output.
Also, sample measuring apparatuses sometimes switch the output of a laser diode in accordance with the measurement mode. The specimen analyzer described in Patent Literature 1 uses the APC circuit to adjust the current of the drive signal for the laser diode such that a different amount of light can be emitted for each of a DIFF measurement mode, an RET measurement mode, and a PLT measurement mode.
In the case of adjusting the current of the drive signal for the laser diode as above, the specimen analyzer described in Patent Literature 1 needs to perform complicated control via arithmetic processing by a microcomputer in order that the amplitude of the high-frequency signal to be superimposed onto the drive signal of the laser diode can be adapted to the drive signal for the laser diode.